Add value attribute ot PreferenceObject

This provides a mechanism to get the computed preference value.

Fixes #7

README.md

@@ -96,6 +96,7 @@ interface PreferenceManager {
 interface PreferenceObject {
     // null means the preference is not overridden
     readonly override: string | null;
+    readonly value: string;
 	readonly validValues: string[];
 
 	requestOverride(value: string | null): Promise<void>;

index.bs

@@ -184,6 +184,7 @@ If an override is set for this preference:
 [Exposed=Window, SecureContext]
 interface PreferenceObject {
   readonly attribute DOMString? override;
+  readonly attribute DOMString value;
   readonly attribute FrozenArray<DOMString> validValues;
 
   undefined clearOverride();
@@ -202,6 +203,18 @@ interface PreferenceObject {
     1. Return |override|.
 </div>
 
+### {{value}} attribute ### {#value-attribute}
+
+<div algorithm='get preference value'>
+    The <dfn attribute for=PreferenceObject>value</dfn> attribute, when accessed, must run these steps:
+
+    1. Let |preference| be the preference object's name.
+    1. Let |value| be null.
+    1. If an override for |preference| exists, set |value| to the value of that override.
+    1. If |value| is null, set |value| to the UA value of the preference.
+    1. Return |value|.
+</div>
+
 ### {{validValues}} attribute ### {#validValues-attribute}
 
 <div algorithm>